Gas turbine engines conventionally require precisely manufactured shroud rings for maintaining the optimum clearance between the tips of compressor blades or turbine blades and the interior surface of the shrouds over the operating range of rotor speed and temperature variations. Accurate blade tip clearance is required since an over sized gap would reduce the efficiency of a compressor or turbine whereas gap that is too small may result in excessive contact between the blade tips and the interior of the shroud ring.
The necessity of accurate manufacturing tolerance for shroud rings is complicated by the need to accommodate thermal expansion and contraction. Conventional shrouds are often manufactured from forged or cast segments that can expand circumferentially when subjected to extreme heat in the turbine section for example. Cooling air withdrawn from the compressor section also aids in controlling expansion by providing impingement cooling of the shroud segments. Shroud cooling increases component life and operating safety margin. Stress on components and oxidation of materials increases with higher temperatures. To impede leakage between shroud segments, feather seals are usually provided.
A significant disadvantage of conventional shroud manufacture is the high cost of materials and labour involved. Due to high temperatures and the critical nature of the shroud it has been considered necessary to construct shroud segments from forgings of very expensive high temperature alloys. Shroud segments are conventionally produced by machining a significant proportion of the material from thick forged rings. Of the material used to produce the forged rings, approximately 70% of the material is removed during machining operations and converted to scrap metal chips. The high temperature alloys that are used are also of very low machineability. Such complex configurations require use of specialized machine tools, rough machining and finish machining processes as well as grinding of the interior surface adjacent to the blade tips.
The environment within which the conventional shroud segments perform is highly demanding. Reliability and predictable performance of aircraft engines is obviously required since they operate in conditions where failure can result in catastrophe. The importance of critical components such as turbine rotors or turbine shrouds exposed to high rotational speeds, high temperature oxidation, expansion and contraction and vibration have led designers to conventionally specify that critical shroud segments must be manufactured of high strength, high cost alloys in an extremely accurate manner requiring forged components that are accurately machined with great precision.
It is an object of the present invention to provide a lower cost alternative to the manufacture of shroud ring segments from forging of high temperature resistant alloys where a significant portion of the forging is reduced to scrap metal in the machining process.
It is a further object of the invention to produce a shroud ring minimizing the number of axially extending gaps and preferably eliminating gaps altogether.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.